esp32: Adds gcov over JTAG feature

Implements function to dump GCOV data to host via JTAG.
The following functionality was added:
 - Host file I/O
 - GCOV runtime I/O stubs
 - GCOV example

examples/system/gcov/main/gcov_example.c

@@ -0,0 +1,55 @@
+/* Blink Example with covergae info
+
+   This example code is in the Public Domain (or CC0 licensed, at your option.)
+
+   Unless required by applicable law or agreed to in writing, this
+   software is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+   CONDITIONS OF ANY KIND, either express or implied.
+*/
+#include <stdio.h>
+#include "freertos/FreeRTOS.h"
+#include "freertos/task.h"
+#include "driver/gpio.h"
+#include "esp_app_trace.h"
+#include "sdkconfig.h"
+
+/* Can run 'make menuconfig' to choose the GPIO to blink,
+   or you can edit the following line and set a number here.
+*/
+#define BLINK_GPIO CONFIG_BLINK_GPIO
+
+void blink_dummy_func(void);
+
+void blink_task(void *pvParameter)
+{
+    int dump_gcov_after = 2;
+    /* Configure the IOMUX register for pad BLINK_GPIO (some pads are
+       muxed to GPIO on reset already, but some default to other
+       functions and need to be switched to GPIO. Consult the
+       Technical Reference for a list of pads and their default
+       functions.)
+    */
+    gpio_pad_select_gpio(BLINK_GPIO);
+    /* Set the GPIO as a push/pull output */
+    gpio_set_direction(BLINK_GPIO, GPIO_MODE_OUTPUT);
+    while(dump_gcov_after-- > 0) {
+        /* Blink off (output low) */
+        gpio_set_level(BLINK_GPIO, 0);
+        vTaskDelay(1000 / portTICK_PERIOD_MS);
+        /* Blink on (output high) */
+        gpio_set_level(BLINK_GPIO, 1);
+        vTaskDelay(1000 / portTICK_PERIOD_MS);
+        blink_dummy_func();
+    }
+
+    // Dump gcov data
+    printf("Ready to dump GCOV data...\n");
+    esp_gcov_dump();
+    printf("GCOV data have been dumped.\n");
+    while(1);
+}
+
+void app_main()
+{
+    xTaskCreate(&blink_task, "blink_task", configMINIMAL_STACK_SIZE, NULL, 5, NULL);
+}